1. Field of the Invention
The present invention relates to a method for determining the sensitivity of a sensor, e.g., an acceleration sensor or magnetic field sensor.
2. Description of the Related Art
Magnetic field sensors (magnetometers) are used as sensors for detecting the earth's magnetic field for compass applications. Microelectromechanical sensors (MEMS) are known which, using the Lorentz force, convert a magnetic field which is present to a mechanical deflection, and capacitively read out same. For this purpose, these sensors have electrical conductors which extend, at least in places, perpendicular to the magnetic field to be measured. Published German patent application document DE 198 27 056 A1 describes such a magnetic field sensor.
In addition, acceleration sensors are known which are based on capacitively readable microelectromechanical systems (MEMS).
The sensitivity of microelectromechanical sensors is based on various parameters, which may be controlled with varying degrees of success in the manufacturing process and during operation.
The sensitivity of microelectromechanical sensors is influenced, among other factors, by the “edge loss,” the difference between the design width of a structure in a functional layer and the structural width in the functional layer which is actually achieved after processing. Edge loss may result, for example, in reduced spring stiffness, seismic mass, and useful capacity for the capacitive evaluation.
For microelectromechanical magnetic field sensors, the sensitivity is influenced in particular by the following parameters:                the current required for producing the Lorentz force; the accuracy of this current is defined primarily by the evaluation circuit;        the mechanical sensitivity of the sensor, which is essentially determined by the mechanical stiffness of the spring system, which in turn is a function of:                    process-related variations in the edge loss (width of the spring and/or the total mass) and            the thickness of the functional layer used (thickness of the spring and/or total mass); and                        the electrostatic sensitivity of the sensor, which varies significantly due to fluctuations in the distance between the electrodes.        
For magnetic field sensors which measure a magnetic field component perpendicular to the substrate plane of the sensor (Bz elements), fluctuations in the edge loss essentially result in variation in the distances between the electrodes, and thus result in variation in the base capacitance. Depending on the evaluation principle used (ΔC or ΔC/C), the thickness of the functional layer may also have an influence. For a ΔC evaluation the electrostatic sensitivity is a function of the thickness of the functional layer. However, since dependencies in the elasticity and the capacitance compensate each other, the overall sensitivity is independent of these parameters. In contrast, for a ΔC/C evaluation the electrostatic sensitivity is independent of the thickness of the functional layer. However, due to the spring constant, the overall sensitivity is a function of the functional layer thickness.
For magnetic field sensors which measure magnetic field components parallel to the substrate plane of the sensor (Bx/By elements), the electrostatic sensitivity essentially varies as a function of fluctuations in the thickness of the sacrificial layer, which defines the distance between the evaluation electrodes.
Thus, the main causes of variations in the sensitivity of microelectromechanical magnetic field sensors and acceleration sensors are in particular process-related fluctuations in the edge loss, the functional layer thickness, and the sacrificial layer thickness.
These fluctuations may be determined by external mechanical stimulation of the sensors, and then adjusted.
Magnetic field sensors are usually adjusted at the end of the production line. The magnetic field sensors are exposed to a well-defined, homogeneous, external magnetic field of known magnitude, the sensitivity and the zero error (offset) are ascertained, and then brought to the desired value by correction of parameters internal to the sensor, for example offset bits. Other characteristic curve parameters, such as the temperature coefficient of the offset (TKO) and of the sensitivity (TKE), as well as nonlinearities, etc., may also be adjusted. In that case, however, the process is more complicated.
To allow a mechanical sensitivity adjustment to be dispensed with it is necessary, among other things, to have the most accurate knowledge possible concerning the edge loss.
Published German patent application document DE 10148585 A1, U.S. Pat. No. 5,618,989, and U.S. Pat. No. 6,840,106 B1 propose methods for checking the functionality of acceleration sensors.